Phase 7: E2EE support

Implements PLAN.md Phase 7: the server half of end-to-end encryption.
Devices upload identity, one-time and fallback keys; other devices query
and claim them, across servers too, and every one-time key is handed out
exactly once. Cross-signing, signature uploads, device list tracking and
server-side key backup complete the set.

internal/keyserver is the component. Keys are stored in canonical JSON
so that re-uploading the same key is a byte comparison, and signatures
uploaded later are kept apart from the key they sign and merged in on
read, so a key's own JSON stays what its owner uploaded. Device keys must
be for the caller's own device; cross-signing keys must be the
uploader's, carry the right usage and, for self- and user-signing keys,
be signed by the master key. Signatures are verified against the key as
stored. Replacing a master key asks for the password; a first upload or
an upload of the keys already stored does not, as the spec allows. A
signature of another user's master key is shown only to the user who
made it and never federated.

Every write that changes what /keys/query answers moves the device list
stream in the same transaction, one position per device, and wakes the
syncs of everyone sharing a room. The stream is the sixth field of a sync
token; older tokens still parse. The syncapi works out device_lists
changed and left from the stream and from membership moves in the
window, skipping invites and knocks, and reports
device_one_time_keys_count and device_unused_fallback_key_types, the
latter as [] when empty. GET /keys/changes uses the same logic.

Over federation: /user/keys/query, /user/keys/claim and
/user/devices/{userId}, and m.device_list_update and m.signing_key_update
both ways. Remote keys are not cached; an update about a remote user
sharing a room moves them in the stream so local clients re-query.
Device display names are not sent to other servers. With federation off,
remote users are reported in failures at once.

Deleting a device deletes its keys, through a new userapi
DeviceObserver. Key backup keeps the better copy of each session by the
spec's rule, never reuses a deleted version's number, and answers writes
to a stale version with M_WRONG_ROOM_KEYS_VERSION.

Storage is migration 0008 for sqlite and postgres, with a PocketBase
stub. The Complement target sets gain the key, device list, fallback,
cross-signing and backup tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/internal/storage/sqlstore/sqlstore.go

@@ -61,6 +61,7 @@ type Store struct {
 	sync     *Sync
 	fed      *Federation
 	media    *Media
+	keys     *Keys
 }
 
 // New builds a Store. It does not touch the database.
@@ -81,6 +82,7 @@ func New(opts Options) (*Store, error) {
 		sync:     NewSync(opts.DB, opts.Rebind, opts.IsUniqueViolation),
 		fed:      NewFederation(opts.DB, opts.Rebind),
 		media:    NewMedia(opts.DB, opts.Rebind, opts.IsUniqueViolation),
+		keys:     NewKeys(opts.DB, opts.Rebind),
 	}, nil
 }
 
@@ -114,6 +116,9 @@ func (s *Store) Federation() storage.FederationStore { return s.fed }
 // Media implements storage.Database.
 func (s *Store) Media() storage.MediaStore { return s.media }
 
+// Keys implements storage.Database.
+func (s *Store) Keys() storage.KeyStore { return s.keys }
+
 // Migrate implements storage.Database.
 func (s *Store) Migrate(ctx context.Context) error {
 	ms, err := s.migrations()